As integrated circuit chips become more complex, increasing use is made of hybrid integrated circuit (HIC) technology, in which a ceramic substrate, upon which relatively complex printed circuitry can be formed, is used to support and interconnect one or more integrated circuit chips. The HIC is usually encased in a protective encapsulation. While this encapsulation may be a hard plastic package, increasing use is being made of silicone resins as an encapsulating medium.
As described, for example, in the U.S. patents of Wong, U.S. Pat. Nos. 4,508,758, granted Apr. 2, 1985, No. 4,396,796, granted Aug. 2, 1983, and No. 4,552,818, granted Nov. 12, 1985, each of which is assigned to a subsidiary company of the American Telephone and Telegraph Company, Inc., a particularly attractive encapsulate is known as RTV (for room temperature vulcanized) silicone. Such silicones are useful encapsulants because of their good thermal stability, good dielectric properties, chemical stability and resistance to atmospheric deterioration. In its uncured state, RTV silicone is fairly fluid and flows about semiconductor chips mounted on a ceramic substrate to completely encase such chips. If the chips are connected to the conductor pattern of the substrate by tiny bonding wires, RTV silicone is capable of flowing about such bonding wires and encasing them without breaking them. After curing, the silicone is sufficiently hard to protect the chips from the hazards of routine use as well as from atmospheric contaminants.
The printed circuit conductors on the surface of the ceramic substrate are typically terminated by lead conductors extending in a cantilever fashion from opposite sides of the substrate. When wired into a circuit, the leads typically constitute a mechanical support for the HIC as well as being electrical conductors.
Most of the assembly functions in modern electronic technology are performed by automated apparatus. Thus, for example, uncured silicone is dispensed onto a substrate surface as part of an assembly line operation, and it is intended that it initially be contained on the substrate by surface tension. After exposure to room temperature conditions for a few minutes, the RTV begins to cure, and after a longer time period, it reaches its final state of mechanical stability. One problem that has been confronted is the tendency of the uncured silicone to flow along the conductive leads and become hardened to the leads during subsequent cure. It is usually intended that the leads be free of contaminants and not insulated so that they can be conveniently and dependably connected to other apparatus such as printed wiring boards, again typically through automated procedures. Thus, silicone that has flowed onto the leads must normally be manually removed prior to subsequent processing.
Masking tape over the leads has been used to reduce the problem of silicone flow onto the leads, but has usually been found to be less than satisfactory. It is difficult to get a masking tape to adhere well to the individual leads without resort to an adhesive that would leave harmful contaminants on the leads after the tape is removed. Conventional masking tape employing a conventional adhesive tends to lift off from the leads at various locations prior to deposition of the silicone. We have found that the fluid silicone then tends to flow between the masking tape and the leads by capillary action at locations at which there is not complete adhesion of the tape to the leads. Even with complete adhesion, the silicone sometimes flows over the masking tape by capillary action, thereby tending to drain the surface of the substrate of the fluid silicone.